This invention relates to methods and compositions for protecting agricultural and horticultural land from damage and infestation by molluscs.
Terrestrial pulmonate gastropods, slugs and snails (collectively molluscs) are significant plant pests that affect commercial agriculture, horticulture and domestic gardens. These molluscs are omnivores and consume large amounts of vegetative material during their daily foraging. Consequently, they can seriously damage vegetable gardens and even plant crops during all phases of the plant growing cycle. The degree of damage they cause is dependent upon the activity and density of the molluscs and on their feeding habits. Each species of mollusc has feeding preferences that differ. For example, Arion rufus (L.) prefers the Solanaceae and Compositae, while Deroceras reticulaturn (Muller) selects Leguminosae, Cruciferae, Compositae, carrot roots, potato leaves and tubers, cereals and seedlings. Arion hortensis Fer seeks out living plants, carrot roots, tubers and bulbs. Control measures are necessary to ensure adequate protection of growing plants from the destructive potential of molluscs.
A wide variety of approaches have been attempted to control and combat pests or molluscs. Perhaps the most common is the use of poisonous compounds called molluscicides. Molluscicides can comprise a variety of chemical compounds including table salt (NaCl), calcium arsenate, copper sulfate and metaldehyde. Metaldehyde, synthesized by the polymerization of acetaldehyde in suspension in ethanol in the presence of an acidic catalyst, has become the most common molluscicide in commercial use.
A major drawback of most molluscicides is that they are contact killers and that they have little effect if molluscs do not come into direct physical contact with the molluscicide, Thus, if a sprayed molluscicide does not contact molluscs, or if molluscs migrate into an area after spraying, the molluscicide typically does not affect the molluscs, This lack of residual action is a major problem for many molluscicides,
Metaldehyde, however, has the advantage of providing contact toxicity as well as acting as an ingested toxin. As a result, this compound is useful as a long lasting bait, serving to attract the molluscs and to kill them after ingestion of the compound. Despite its high degree of effectiveness and its commercial popularity, metaldehyde is toxic to higher mammals and is a major, worldwide contributor to domestic animal poisoning.
Mollusc control is also effected by mechanical trap devices. These devices are of a variety of designs, including those described in U.S. Pat. Nos. 4,821,452 and 4,747,230. These devices are generally designed to attract molluscs and then entrap them. Mortality results either by molluscicide baits or by watertraps that drown the molluscs. Although traps can be effective, they require continual tending to remove dead molluscs and to add fresh bait. Another drawback of such devices is that they are effective only over a relatively small area.
Chemical and/or mechanical barriers also are effective mollusc control devices that bar mollusc entry into a protected area. A typical mechanical barrier, described in U.S. Pat. No. 4,756,116, consists of a pair of dissimilar metal strips that cause an electrolytic charge when the molluscs attempt to cross the strips. Another example of a mechanical barrier is copper strips that are placed around trees or flower beds. Chemical barriers are usually compounds that are highly irritating to molluscs and deter them from crossing the barrier. Known chemical barriers include lime, fine sawdust, diatomaceous earth, ash, egg shells, metaldehyde, and table salt. One problem inherent with known chemical barriers is their lack of residual action. To be effective these barriers must have sufficient solubility to dissolve in the molluscs' proteinacious slime. However, this solubility can also cause the barrier to disintegrate quite rapidly in the warm, moist environments that molluscs commonly inhabit. The result is that the chemical barriers quickly lose their potency. To offset the tendency of chemical barriers to disintegrate in the environment, chemical barriers have sometimes been embedded in a long lasting matrix such as a polymer. One commercial product known as "Slug and Snail Defense" sold by C. D. McGarr Enterprises, consists of a sodium chloride-impregnated plastic barrier.
Accordingly, improved chemical barriers are needed to control and prevent damage by molluscs to crops, plants and trees.
Accordingly, an object of the invention is to provide a chemical mollusc barrier that is both effective and environmentally compatible. Another object of the invention is to provide such a barrier having sufficient solubility to irritate and repel molluscs, and having sufficient ability to resist environmental degradation to be effective for a suitable length of time. A further object is to provide such a barrier that can be economically used to effectively protect sizable agricultural or horticultural plots. Other objects of the invention will be apparent from the disclosure that follows.